Serrated magnetic random memory cell and means for connecting a pair of adjacent cells

ABSTRACT

A random access thin film magnetic memory is provided. Individual memory elements, characterized by serrated edges, are separated by connecting regions. Each connecting region has a polygonal-shaped opening therein with a first pair of adjoining edges having a length longer than the lengths of the remaining adjoining edges. Each memory element may be independently addressed through overlying row and column conductors to either read or write data into the element, and may contain a logic &#34;zero&#34; or &#34;one&#34; determined by the state of the magnetic domain found in the element.

BACKGROUND OF THE INVENTION

The present invention relates to magnetic memory cells used to storedigital electronic data, and more particularly to an improved magneticmemory cell which functions as a random access memory.

The dipole magnetic moments of neighboring atoms within a small region,or domain, of a thin film of magnetic material align themselves whenplaced in a sufficiently strong external magnetic field. This alignmentof magnetic dipole moments is unique to magnetic materials (Fe, Co, Ni,Gd and Dy) and takes place despite the random motion generally undergoneby atoms within any material. The material orientation of the magneticdipole moments remains after the external magnetic field is removed.

Transition regions exist between any two domains which do not have thesame alignment of magnetic dipoles. The transition regions between suchdomains are called domain walls. Different types of domain wallstypically exist in magnetic material, each unique as to the orientationof the magnetic field existing within or comprising the domain wall.Within the type of domain wall referred to as a Neel wall, the magneticfield rotates in the plane of the film 180°, thus separating twoantiparallel domains. Reversing the magnetic field direction in a smallportion of a Neel wall results in the creation of a different type ofdomain wall, the cross tie. The cross tie magnetization constitutes aseparate stable magnetic domain.

The characteristic magnetic fields of the domain wall types remainsunchanged in the absence of an external magnetic field of apredetermined strength. In the presence of an external field of thepredetermined strength, however, the magnetic state of a domain wall atany given location can be changed.

The stable magnetic domain states of the magnetic film represented bythe domain wall magnetization fields may be utilized within a memorysystem for the storage of digital data. U.S. Pat. No. 3,868,659, issuedon Feb. 25, 1975 to Leonard J. Schwee, discusses the use of thin filmmagnetic materials as data storage devices. A more recent disclosure ofsuch use of thin film magnetic materials is contained in U.S. Pat. No.4,246,647, which on Jan. 20, 1981 to Johnson et al. In both of thosepatents, the memory disclosed is a serial memory, i.e., once a data bitis entered at one end of the memory, it is passed through the memory andcannot be removed until all data entered ahead of it has been removed.The operation of such a memory is fully described in theabove-referenced patents. Such a serial memory has obvious limitations,in that it is often desirable to randomly access data which has beenstored in the memory. In a serial access memory, to access a given databit, it is necessary to first read out all data which was entered beforethe data bit of interest.

Long serial access type magnetic memory systems, such as the onesdescribed above, are subject to drawbacks in that the magnetic fieldswithin the thin film strips are easily disturbed by the ambient externalmagnetic fields which exist in the area surrounding the magnetic memorysystem, thus disturbing the location of the Neel walls or the cross-tiesand the data these features represent. The lengthy Neel walls whichexist in such serial memories are particularly easy to disturb, and havea tendency to move from the center of the magnetic strip to one side onthe other of the strip. As a result of these disadvantages, there aremany opportunities for error in reading and writing into such a memory.

SUMMARY OF THE INVENTION

One object of the present invention is to overcome the problems anddisadvantages of the prior art serial access memories by providing arandom access magnetic thin film memory system within which each memoryelement may be randomly accessed to either read or change the data bitstored upon it.

It is an additional object of the present invention to provide a randomaccess magnetic thin film memory system in which there is less chancefor error in reading and writing into a particular memory cell.

It is a still further object of the present invention to provide amagnetic thin film memory system on which the effects of ambientelectromagnetic fields may be minimized.

Additional objects and advantages of the invention will be set forth inthe description which follows; and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the random accessmagnetic thin film memory system of the present invention comprises athin strip of magneto-resistive material disposed upon an insulatingsubstrate, a plurality of memory elements defined within the thin filmby having a serrated edge pattern thereto, with each memory elementbeing separated from the other memory elements by a non-serratedconnecting region of the magneto-resistive material having a voidtherein, the system having a plurality of spaced conductor meansinsulated from and overlying the memory elements for detecting orchanging the memory state of those memory elements and havingconventional control electronics connected to the spaced conductor meansto read data stored in or to write data into the memory.

The accompanying drawings, which are incorporated in and constitute apart of this application, illustrate one embodiment of the inventionand, together with the description, serve to explain the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly magnified view of a portion of a serrated randomaccess memory (SRAM) according to the present preferred embodiment ofthe invention;

FIG. 2 is a highly magnified fragmentary view of the memory of FIG. 1illustrating a portion of the film strip of magneto-resistive material;

FIG. 3(a) is a highly magnified view of one serrated portion of onememory bell of the present invention, illustrating the alignment of themagnetic dipoles with a strong magnetic field perpendicular to themagnetic film strip;

FIG. 3(b) illustrated the alignment of the dipoles in FIG. 3(a) when thestrong magnetic field is removed;

FIG. 4(a) is a highly magnified view of one desired configuration of oneof the connecting regions showing the alignment of the magnetic dipolesin that region with a strong magnetic field oriented perpendicular tothe magnetic film strip;

FIG. 4(b) illustrates the alignment of the dipoles in FIG. 4(a) when thestrong magnetic field is removed;

FIG. 5 illustrates an alternate desired configuration of one of theconnecting regions; and

FIG. 6 is a graph showing the relationship of the resistivity of amemory cell of the SRAM of FIG. 1 as a function of the strength of anapplied magnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, which is a highly magnified view of a portionof a SRAM according to the present invention, a description of a randomaccess memory constructed according to the present invention will bedescribed. FIG. 1 illustrates a plurality of strips 10 of magneticmemory material, each containing a plurality of memory cells 12 andconnecting regions 32. The memory cells 12 are arranged in rows A B andC and columns 1, 2 and 3. Also shown are row conductors 14 which passover each memory cell 12 in a row and which are connected to row decoder16. Column conductors 18 are also shown which pass over each memory ell12 in a column and which are connected to a column decoder 20.

Referring to FIG. 2, two memory cells 26 and 28 from a memory systemaccording to the present invention are illustrated. The memory cells areconstituted by the areas of the thin film strip 10 having serrated sideor edge portions 30 with one edge of each serration longer than theother. The serrations are preferably shaped so that if they wereextended to the centerline 40 of the strip of magnetic material 10,angles θ₁ and θ₂ would be formed, as shown in FIG. 2. Angle θ₁ shouldalways be an acute angle, while angle θ₂ should always be an obtuseangle. The value of θ₁ should vary between 0° and 90°, while the valueof θ₂ is equal to 180° minus θ₁. Typically, the memory element ispreferably 15 micrometers wide from one serration tip to the other, and8 micrometers side at its narrow point, under the serrations. Themagnetic strip may be typically 400 Angstroms thick, for example. FIG. 2illustrates five serrations per memory cell, but any number, e.g., fromtwo to ten or more, may be employed. As will be explained later, thenumber of serrations employed will effect the memory element'sresistance to the effects of ambient magnetic fields, and will alsodetermine the signal level needed to read or write data into the memoryelement.

Between the two memory elements 26 and 28 is a non-serrated connectingregion 32 which may be a polygonal shaped portion of the magnetic filmwith an internal void 34 existing therein. In accordance with theembodiment of FIG. 2, void 34 is formed in the shape of a polygon,preferably a quadrangle. Connecting region 32 is formed with two longadjoining sides 55 and 57 of equal length and two shorter adjoiningsides 59 and 61 of equal length to form internal void 34 having two longadjoining edges 56 and 58, and two short adjoining edges 60 and 62, suchthat void 34 is symmetrical along one axis and non-symmetrical along theother axis. Long adjoining edges magnetic strip 10. Angles θ₃ and θ₄ areapproximately equal to angles θ₁ and θ₂ as illustrated in FIG. 2.Therefore, long edges 56 and 58 of void 34 of connecting region 32 areapproximately parallel with serrations 30 of memory element 26.

In FIG. 2, the upper memory element 28 illustrated shows the presence oftwo domain states 36 and 38, which may be chosen to represent either alogic "1" or a logic "0." For purposes of this description, a logic "1"will be represented by the presence of both domain states (a cross tieand a Neel wall). The lower memory element 26 shows the presence of onlyone domain state 36, and thus contains a logic "0." Domain state 36 iscommonly referred to a Neel wall. Domain state 38 is commonly referredto as a cross-tie. In the memory elements of the present invention, across-tie 38 will be present across the narrow portions of each memoryelement as shown in FIG. 2, when a logic "1" is stored in that element.A description of how Neel walls and cross-ties are formed within amemory element will now be provided.

When the memory cells illustrated in FIG. 2 are first formed, themagnetic dipoles present in the thin film material can be oriented inrandom pattern, as they normally are in all magnetic material.Therefore, before use of the cells as memory elements, all the dipolesmust be oriented in a consistent manner. FIGS. 3(a) and 3(b) illustratethe initial alignment of the magnetic dipoles. A strong externalmagnetic field 50 is oriented perpendicular to the magnetic strip 10,which causes the magnetic dipoles 52 to align themselves with themagnetic field, as shown in FIG. 3(a), which illustrates only oneserration portion of a memory cell, but which is exemplary of all cellsand the connecting regions. Once the strong external magnetic field 50is removed, the magnetic dipoles 52 will rotate into a stable state, thedirection of rotation determined by the smallest angle through which thedipoles 52 must pass to return to the stable state. That rotation isillustrated in FIG. 3(b) by where the dipoles located in the upper halfof the strip have rotated to the left, whereas the dipoles located inthe lower half of the strip have rotated to the right, as viewed in thedrawing. This results in orientation of the dipoles within the strip asshown in FIG. 3(b). The border line or division 36 between the dipolesis referred to as a Neel wall.

A long Neel wall in the middle of a thin film magnetic strip constitutesa very delicate equilibrium condition. As a result, in prior art thinfilm magnetic strips, in the presence of electronic noise orelectromagnetic fields the Neel wall tends to move away from the centerof the film, and perhaps attach itself to a side edge of the thin filmstrip. The present invention solves those problems by providing aconnection region 32 between each memory element, and thus allowing forvery short Neel walls to be formed, which are less subject todisruption. As shown in FIG. 2, the Neel wall 36 which is formed in eachmemory element attaches between the two points 42 formed by the void 34in the connecting region 32. This results in the required Neel wallbeing very short, and greatly minimizes any chance that the Neel wallwill become improperly oriented with respect to the thin film strip.

The orientation of the connecting region 32 between memory cells 12 iscritical in increasing the stability of the domains within the thin filmstrip. Experimentation has shown that although void 34 is necessary tokeep to a minimum the length of the Neel walls formed in the memoryelements, the void itself can lead to improper magnetic dipole alignmentin the connection regions, and thus lead to the disturbance of themagnetic field within the memory cells themselves, if the void isimproperly shaped.

Experimentation has shown that for best results the void 34 should beshaped substantially as shown in either FIG. 4, or FIG. 5. When void 34is formed in a quadrilateral shape with two long edges 56 and 58adjoining two short edges 60 and 62, as shown in FIG. 4(a), the dipolesalign themselves correctly upon relaxation of the initial strongmagnetic field so that the field forces do not oppose each other in theconnecting region. See FIG. 4(b). The domains in the shorter sides 59and 61 of connecting region 32 are forced into proper alignment by thestrength of the field in the longer sides 56 and 58. Therefore, nointernal disturbances are created in the magnetic field. FIG. 5 shows anacceptable alternative shape for void 34.

As embodied in FIG. 5, a connecting region 32' has a void 34' withelongated adjoining edges 56' and 58' similar to void 34 of connectingregion 32, previously described. Although void 34' is in the form of anonsymmetrical polygon as void 34 of FIG. 2, void 34' has two adjoiningshort edges 64 and 66 forming an obtuse angle at point 42'.

Thus, in FIG. 4, the angle θ₅ formed where the long edges 56 and 58 joinshort edges 60 and 62 are greater than 90 degrees, whereas angles θ₅ inFIG. 5 are less than 90 degrees; and the internal angle formed byshorter sides 64 and 66 is greater than 180 degrees. This alternativeshape will also result in proper alignment of the magnetic dipoleswithin the connecting region. The value of angles 0-5 is not critical,so long as points such as 42 and 42' result, to which the Neel wallsattach, as discussed earlier.

As shown in both FIGS. 4 and 5, not only must void 34 and 34' be formedin an elongated fashion, but it must also be formed so that long edges56, 56' and 58, 58' 30 in the memory cells. This allows the angles θ₁and θ₃ to be approximately equal, as discussed earlier. In this manner,the magnetic dipoles in both the memory cells 12 and the connectingregions 32 relax in the same direction after the initializing strongmagnetic field 50 is relaxed, and consistent magnetic domains arecreated.

Reference will now be made to FIG. 6 to explain how data is entered intoa memory element of the present invention, and how the data contained ina memory element is read.

FIG. 6 shows the resistivity of a memory element plotted against thestrength of a magnetic field applied to that element, and illustratesthe magneto-resistive effect which is used to either read or write intoa memory element. As the graph illustrates, the resistivity of any givenmemory element depends upon two factors, the state of the magneticdomains within the element, i.e., whether both a Neel wall andcross-ties are present (a data bit "one" is stored in the element), orwhether only a Neel wall is present (a data bit "zero" is stored in theelement), and the strength of an applied external field. When noexternal magnetic field is applied, the resistivity of a memory elementwill be R1 if that element is in the "zero" state and will be R2 if thatelement is in the "one" state.

As discussed earlier the state of a memory element, i.e., whether it isa "one" or a "zero" can be changed if a magnetic field of apredetermined strength is applied to the cell. That magnetic fieldstrength is referred to in FIG. 6 as the WRITE LEVEL. When a magneticfield equal or greater than the WRITE LEVEL is applied to a magneticmemory element in the "zero" state, the magnetic dipoles within theelement reorient themselves, cross-tie magnetic domains 38 are created,and the resistivity of the element changes, as illustrated in FIG. 6. Itis possible to apply a magnetic field of less strength than the WRITELEVEL, which will cause the resistivity of the element to momentarilychange, but which will not cause the state of the memory element, i.e.,"one" or "zero," to change. That lesser strength is referred to in FIG.6 as the READ LEVEL.

With the above principles in mind, a description of how a magneticelement is read will be provided. First, an external magnetic fieldequal to the READ LEVEL is applied to the memory element. As theexternal field is applied, a small D.C. current is supplied to thememory element, and the change in the element's resistivity isdetermined from the voltage developed across the element. As shown inFIG. 6, if the memory element is in the "zero" state, the change inresistivity, from R1 to R3, is small. If the memory element is in the"one" state, however, the change in resistvity, from R2 to R4, is large.Therefore, when a magnetic field equal to the READ LEVEL is applied to amemory element, a large change in the resistivity of the elementindicates it is in the "one" state, while a small change indicates it isin the "zero" state.

The difference in the resistivity change from a "one" to a "zero" stateresults from the different alignment of the magnetic dipoles within thememory element and is maximized as a result of the geometry of theserrated memory elements separated by the connecting regions, asdiscussed above. Therefore, not only does the shape of the memoryelements result in a memory which is more stable and less prone todisruptions due to unintended electromagnetic fields, it also results inan improved ability to detect whether the memory element is in the "one"or "zero" condition.

Referring again to FIG. 6, it may be seen that to change the memoryelement from a "zero" to a "one," all that is necessary is to apply amagnetic field equal or greater than the WRITE LEVEL oriented in aproper direction relative to the cell. Similarly, to change the elementfrom a "one" to a "zero," one simple applies a similar field oriented inthe opposite direction. A "zero" state exists when only a Neel wall ispresent in the memory element, and a "one" state exists when both a Neelwall and one or more cross-ties are present.

In operation, once a random access memory such as in FIG. 1 isconstructed, the individual memory cells must be initialized, oraligned, as is explained in more detail above with reference to FIGS. 3,4 and 5. This is accomplished by applying a large magnetic fieldperpendicular to the columns of memory cells. This field is created withexternal field coils which generate a perpendicular magnetic field toinitially align the magnetic moments in the magnetic material and createthe Neel walls in each memory element 12 between each connecting region32.

Data may then be written into the random access magnetic memory systemof FIG. 1 as follows. First, the process circuitry of the memory systemdetermines that a data bit should be placed in a specific memory cell.That information is passed to column decoder 20 and row decoder 6, whichwill determine, for example, that the memory cell located in column 2,row B has been addressed. Current will be passed through row B conductor14 and column 2 conductor 18. The current sent in either the row orcolumn conductor is not enough to generate an external magnetic field ofthe WRITE LEVEL strength of FIG. 6. However, because the memory elementfound in column 2, row B is subject to the magnetic fields generated byboth the row B conductor and the column 2 conductor, the magnetic fieldover that element is equal or greater than the WRITE LEVEL and the databit is changed. As one of ordinary skill in the art will realize, onlythe direction of the currents must be changed to write either a "one" ora "zero" into any given memory location. To read any given memorylocations, it is only necessary to pass current through either of therow or column conductors to generate a field of the READ LEVEL strength,and thus cause the change in resistivity discussed earlier.

The amount of current required to generate a magnetic field ofsufficient strength to change the state of a memory cell is directlyrelated to the number of serrations in that cell. As explained earlier,the number of serrations formed within each memory cell may vary from 2to 10 or more. As the number of serrations increases, the number ofcross-ties which must be created or destroyed by the magnetic field alsoincrease, thus requiring a stronger field. Therefore, the memory of thepresent invention may be custom designed to withstand varying degrees ofambient electromagnetic fields without damage to the data stored in it.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the random access memory ofthe present invention without departing from the spirit or scope of theinvention. It is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended clams and their equivalents.

What is claimed is:
 1. A magnetic random access memory comprising:aninsulating substrate; a thin film of magneto-resistive material disposedupon said substrate, said thin film having a plurality of spaced memoryelements therein; each said memory element being defined by a serratededge pattern; each said memory element being spaced from the other saidmemory elements by a non-serrated connecting region having a void in themagneto-resistive material, said void defining a polygon-shaped openingin said connecting region symmetrical about one axis thereof andnon-symmetrical about an axis substantially perpendicular thereto; and aplurality of spaced conductor means, insulated from and overlying saidmemory elements, for selectively detecting and changing the memory stateof any selected one of said memory elements.
 2. A magnetic memory as inclaim 1, wherein the number of said serrations in said edge pattern ofeach said memory element determines the amount of energy required todetect or change the state of sad memory element.
 3. A magnetic memoryas in claim 1 wherein the number of cross-tie magnetic domains which maybe created is determined by the number of said serrations in each saidmemory element.
 4. A magnetic memory as in claim 1, wherein there aretwo of said spaced conductor means overlying each of said memorylocations, one of said conductor means representing the row in whichsaid memory location is found, and the other of said conductor meansrepresenting the column in which said memory location is found.
 5. Amagnetic memory as in claim 4, wherein one of said conductor means isenergized to detect the state of said memory cell and both of saidconductor means are energized to change the state of said memory cell.6. A magnetic random access memory comprising:an insulating substrate; athin film of magneto-resistive material disposed upon said substrate,said thin film having a plurality of spaced memory elements therein;each said memory element being defined by a serrated edge pattern; eachsaid memory element being spaced from the other said memory elements bya non-serrated connecting region having a polygonal configured void witha pair of adjoining edges longer than the remaining adjoining edges,said first pair of adjoining edges extending substantially parallel toone edge of each said serration and joined at a point central of saidserrated edges.
 7. A magnetic memory as in claim 6 wherein said void isquadrangular in configuration.
 8. A magnetic memory as in claim 7wherein said void has a diamond configuration symmetrical along one axisand non-symmetrical along another.
 9. A magnetic memory as in claim 7wherein said remaining adjoining edges comprise a pair of edgessubstantially equal length joined at a point central of the serratededges, said edges forming an internal angle at said point greater than180 degrees.
 10. A magnetic memory as in claim 6 wherein the serratededge pattern includes serrated edges each having one edge of eachserration longer than the other.
 11. A magnetic memory as in claim 7wherein the thin film of magneto-resistive material has a width ofapproximately fifteen micrometers from the tip of one serrated side edgeto the other.